Well I'll be damned, looks like 5MHz mode on Mikrotik is just slower
signalling! All 52-subcarriers are indeed there, although a little hard
to see:
If you count peaks left/right of center, you'll get 26. 2*26=52, so
that's every sub accounted for.
Can you point me at your 5/10MHz docs? And which analog filter you're
referring to?
--Bart
On 8/27/2014 4:47 PM, Adrian Chadd wrote:
On 27 August 2014 16:38, Bart Kus <m...@bartk.us> wrote:
I'm guessing the chip generates its own internal clocks from an external
reference. Can that PLL be slowed down ahead of timers overflowing?
Hopefully the PCI clock is generated independently.
Yeah, that's what you're slowing down - you program the PLL.
The PCI clock is generated separately.
Also, I think Mikrotik implements narrower bands by dropping subcarriers
instead of slowing down their symbol rates. I'll try to get a good spectrum
picture of their 5MHz mode tonight. Keeping the subcarrier symbol rates
relatively higher might offset some analog droops, at the cost of OFDM skirt
sharpness.
Hey cool, if they're doing that then I should likely go digging for
the PHY documentation for the AR5414 and find out.
Also, a slight correction. I think you meant the subs are 312.5kHz wide,
which would result in a 200kHz emission having 3.125kHz wide subs. Or,
perhaps more realistically, running at 1/128th the speed, 2.44140625kHz
wide. How can you not love a number like that? :)
Someone else can do better math, I'm busy doing non-wifi at the moment. :)
But, it really is 20MHz / 64 carriers == each subcarrier width.
Does the project have a map of all these clocks + timers which might need
tweaking for spectrum reduction? I know you can't cite original Atheros
docs, but perhaps there's been derivative documentation works created?
Not at the moment I'm afraid. I haven't really dug into that level of
detail. I documented what's needed for 5 and 10MHz modes.
However - there's an analog filter that I don't know if it's
programmable or not. It's used for both transmit and receive
filtering. I know on these chips it can do 5/10/20/40 but I don't know
if it's an arbitrary width.
-a
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